"This irritates me a lot more than it should. Adult women in books set in the present day being called little-girl names like Evie/Ava/Lexi when in reality they’d be called Sarah, Emma etc."
I disagree with this - to start with, if you're talking about adult women aged between 18 and 100, there's a big age range there - 82 years, and up to 5 generations. Then there are influences from different cultures etc.
Lexi is short for Alexandra - in my family there are two, aged roughly 32 and 7. Shortened to Alex rather than Lexi. Family of family also includes a Sandy in my parents generation/age band, and that might well be short for Alexandra. It's also a name that's common across different cultures as either a main name or an English name (in families who give kids names from more than one culture), and I've met women with versions of the name from Argentina and Nigeria.
I also know an Ava in her 40s, and if anything I'd expect it to be quite common among women slightly older than my friend. They could be named after the film star Ava Gardner.
Evie could be a diminutive of Evelyn, Eve or Eva. It's another name which might well be used in a variety of cultures - English, Irish, western, central and eastern Europe. I wouldn't think anything of meeting a woman of any age called Evie And there are lots of names that started as a diminutive of a more formal name but have been given as names in their own right - Sally (Sarah), Jenny, Polly, Molly (both Mary), Sasha (could be Alexandra or Sarah).